This application is directed towards an improvement over U.S. Pat. No. 3,800,576 to Barrett et al., and U.S. Pat. No. 4,626,007 to Larson, which are incorporated herein by reference.
Barrett discloses a combination lock having shiftable combination rings for re-programming. The rings are rotatably mounted with a knob, and each defines a notch. The notches are engaged by a set of checkers on a bolt when the rings are aligned in the proper combination so that the bolt may be retracted. The bolt is spring biased to the extended position, so that it engages the edge of an aperture in a key box secured by the lock. The bolt has several parts that slide relative to one another, allowing the extended bolt to cam against the edge of the aperture. The extended portion of the bolt thereby retracts, permitting the keybox to be closed and locked when the combination rings are in a scrambled, locked position.
Several of the limitations in the prior art relate to the security and tamper-resistance of the existing locks. Although no lock is perfectly secure or tamper-proof, locks produced according to the above-referenced patents are extremely effective and are widely used. Nonetheless, a reduction in an already small security risk is desireable.
A drawback of the prior art multi-part spring biased bolt is that the lock may be closed and locked after re-programming without the user having confirmed the newly re-programmed combination. Thus, an error in programming that easily would be detected and corrected before the keybox is closed and locked becomes impossible to correct without damaging or destroying the lock or keybox.
Some spring-loaded bolts may be defeated by skillfully applied external shocks to the lock or housing to dislodge the bolt. Multi-part bolts such as shown in Larson and Barrett may also be defeated by a skillful intrusion into the lock to damage certain critical mechanical elements therein.
Any access to the lock interior may be helpful to an intruder. The prior art locks include a switch button which, although generally well secured, may be removed with skillfully applied extreme force. This may provide an aperture to the interior of the lock. Also, because the spring loaded bolt is not integrally connected with the external switch button, the appearance of the button does not provide a visual indication of a failure of the lock to properly close and latch.
The design of the ring-notch-engaging checkers on existing lock bolts creates other security weaknesses. When the checker end surfaces are generally flat, a skilled intruder may manipulate the rotary knob while applying pressure to the switch button to tactilely sense the positions of the ring notches. To avoid this, pointed checkers have been used in conjunction with an array of shallow false notches on the combination rings to make such lock picking nearly impossible. However, sharply pointed checkers may be wedged into the rings by pounding on the switch button, which may drive the bolt sufficiently far to release the lock.
From the foregoing it will be recognized that there is a need for a combination lock that overcomes these drawbacks of the prior art.
In accordance with the preferred embodiment of the present invention, this need is fulfilled by providing a unitary bolt controlled by an over center spring so that the bolt is stable in both the extended and retracted positions. The combination rings are removable for reprogramming only when the bolt is in the extended position. In this position, however, the bolt prevents the door from being latched to the box. To retract the bolt, the combination must first be dialed properly. The lock's switch button is directly engaged with the bolt so that it provides a visual indication of the bolt position. The button also includes a weakened section so that the external portion of the button will break away when subject to force, while the internal portion remains attached to the lock to prevent access. The bolt includes ring-notch-engaging checkers, each having a generally flat surface to prevent forcible wedging of the checker into a combination ring, but with each checker also including a small ridge for engaging false notches to prevent picking.